In recent years, in order to obtain radiation images for medical diagnosis, taking the place of conventional systems which use a silver halide film, systems as follows have been brought into practical use. Namely, they are systems which make a phosphor absorb radiation having passed through a photographic object, thereafter excite the phosphor, for example, with light or heat energy, thereby having the radiation energy accumulated in the phosphor radiate as fluorescence, and then detect the fluorescence to generate an image. Specifically, for example, as disclosed in Patent Document 1 and Patent Document 2, there are systems which employ a stimulable phosphor sheet formed with a stimulable phosphor layer on a support body, emit radiation having passed through a photographic object onto the stimulable phosphor layer of the sheet so that radiation energy corresponding to the radiation transmission amount of each portion of the photographic object is accumulated, thereby forming a latent image. Then, the systems scan the stimulable phosphor layer with stimulable exciting light to radiate accumulated radiation energy of each portion and convert it into light, and convert the light intensity into image signals via an optoelectronic converter such as a photomultiplier so as to obtain a radiation image as digital image data.
Visualization of images is carried out by forming an image on a silver halide film, based on the obtained digital image data, or outputting the image data on an image display such as a CRT. Digital image data is sometimes stored in an image storage device such as a semiconductor storage device, a magnetic storage device, an optical disk storage device, or the like, and is later read from the image storage device to be visualized, as necessary.
During scanning a stimulable phosphor sheet with exciting light, the stimulable phosphor sheet and an exciting light source should have precise relative motion at a constant velocity. Patent Documents 3 and 4 disclose radiation image reading apparatuses which perform sub scanning of a stimulable phosphor sheet by moving up and down a member holding the sheet with a belt wound around a driving pulley in the following manner. With high accuracy of positioning and of constant velocity control by the use of a weight balance arranged at the other end of the belt, these apparatuses press a roller (vibration damper) against the belt approximately orthogonally to the conveying direction of the belt to inhibit vibration so that image nonuniformity due to conveying nonuniformity in the sub scanning direction is inhibited.
According to Patent Document 4, a unit for inhibiting the vibration of the vibration damper itself is provided so that image nonuniformity due to conveying nonuniformity in the sub scanning direction is further securely inhibited.
[Patent Document 1] U.S. Pat. No. 3,859,527
[Patent Document 2] TOKKAI-SHO No. 55-12144
[Patent Document 3] TOKKAI No. 2002-278000
[Patent Document 4] TOKKAI No. 2003-98609
However, the above known arts still have problems as follows.
At a start of driving a steel belt from a stopped state, a shock load is applied to the steel belt and vibration damper, requiring a time for stabilization. Therefore, a preliminary conveying space to be used before image reading should be prepared in order to achieve stable sub scanning in image reading. In this case, the motion space as the total of the sub scanning space for image reading and the preliminary conveying space is large, which causes low space efficiency. On the other hand, in a case of starting image reading simultaneously with a start of conveyance-driving without setting a preliminary conveying space, or in a case where enough preliminary conveying space is not allowed, there is a problem that image nonuniformity is caused at a start of image reading.
The time required for stabilization of conveying motion varies with the weight of a conveyed object, the mechanical state of a conveying device, the state of the installation site, and other conditions. If an apparatus is to have a preliminary conveying space that is large enough under any conditions, an even larger motion space is required, causing a problem of even lower space efficiency.